NiSi is employed as a low conductive electrode material in modern semiconductor devices. To form a self-aligned silicide, Ni/Pt is sputtered on the wafer by physical vapor deposition (PVD), and a rapid thermal anneal (RTA) is performed to form rich NiSi phases. Subsequently, excess and unreacted Ni/Pt is stripped with strong acid mixtures such as piranha (a mixture of sulfuric acid and hydrogen peroxide (SPM)) or nitric acid (HNO3) in a sink bath process, followed by a second RTA process to transform the metal rich phases into a low conductive Ni mono silicide. Since unreacted Pt remains after the second RTA, concentrated Aqua Regia (1:4) (HNO3 plus hydrochloric acid (4HCl)) or a hot SPM, for example at temperatures greater than 160° C., is employed to remove the Pt residuals.
The two strip processes must remove the Ni/Pt effectively with a minimal effect on underlying layers. For example, when performing the Ni/Pt strip processes, the etch rate must be small for silicon nitride (Si3N4) spacers, a silicon dioxide (SiO2) field oxide, a TiN high-k metal gate, nickel platinum silicide (NiPtSi) contact electrodes, and nickel platinum silicide germanium (NiPtSiGe) source/drain contact electrodes for strain applications. Although hot SPM is very efficient in removing the Ni and Pt, it also results in a high etch rate for TiN. Weaknesses in the encapsulation can open a path for the SPM to remove the TiN. Therefore, hot SPM is not suitable for a gate first integration scheme.
Aqua Regia works more efficiently for the Pt strip after HNO3 is employed for the Ni strip. When Aqua Regia is applied, the HCl yields chloride ions which react with Pt according to formula 1 to form chloroplatinate ion (PtCl6), which in turn is soluble and forms chloroplatinous acid (H2PtCl4).Pt4+(aq)+6Cl−(aq)→PtCl62−(aq)  Formula 1Accordingly, the Aqua Regia Pt strip process is only effective if the Pt is not bound and can freely react to form the chloroplatinate. If the Pt has formed an oxygenic complex (Pt—O), the reaction with Aqua Regia will not occur, and Pt will remain on the wafer. Neither SPM with a temperature less than 130° C. nor HNO3 in a SNK process for removing unreacted Ni keeps the unreacted Pt in chemical solution, and, therefore, a subsequent application of Aqua Regia will result in Pt residues, which can result in contact-to-gate shorts.
In addition, during middle-of-line (MOL) processes, the spacers are etched back (by spacer proximity technology etch (SPT-ET)) to open a place for tensile plasma enhanced nitride (TPEN) and compressive plasma enhanced nitride (CPEN) strain layers, which are deposited and etched back separately. During SPT-ET, Ni/Pt particles become redistributed and must be removed. Since the distance to the TiN high-k metal gate is short, aggressive etch chemistries can attack the underlying TiN. Therefore, SPM cannot be used for MOL etch passcleans.
Attempts to solve the MOL etch passclean problem have included using a cold SPM, such as at temperatures less than or equal to 90° C., in combination with a standard clean solution #1 (SC-1) spray process (e.g., using a mixture of ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2), and diluted (DI) water). However, underlying TiN may still be attacked.
A need therefore exists for methodology enabling improved removal of Ni/Pt residuals, with low TiN etch rate, and the resulting structure.